Activation of the zymogen plasminogen results in the formation of the fibrinolytically/thrombolytically active serine proteinase plasmin. Activation of endogenous plasminogen can be triggered or enhanced by the administration of a plasminogen activator such as urokinase, streptokinase, staphylokinase or tPA, or any variant thereof. Upon activation, the plasminogen protein is proteolytically cleaved into a heavy chain comprising the 5 kringle domains and a light chain comprising the catalytic domain. Both chains are held together via 2 disulfide bonds. After activation, an autolytic cleavage removes an N-terminal segment from the heavy chain (78 amino acids of human plasmin; 77 amino acids of bovine plasmin) and the bovine plasmin heavy chain can be further autocatalytically cleaved between kringles 3 and 4, hence giving rise to bovine midiplasmin (Christensen et al. 1995, Biochem J 305, 97-102). Activation of plasminogen to plasmin, triggered by the cleavage of the R561-V562 peptide bond in human plasminogen, induces a large conformational change in the light chain, said change resulting in the priming, or activation, of the catalytic triad within said light chain. Bacterial plasminogen activators such as streptokinase and staphylokinase form a complex with plasminogen and, without cleavage of the R561-V562 peptide bond of plasminogen, the catalytic site of plasminogen is activated due to conformational changes upon activator-plasminogen complex formation (plasminogen activation mechanisms are summarized in, e.g., the Introduction section of Terzyan et al. 2004; Proteins 56: 277-284).
Whereas plasminogen activators act as indirect thrombolytic agents, it has alternatively been suggested to use plasmin itself as a direct fibrinolytic/thrombolytic agent. Such direct use is, however, hampered by the fact that plasmin is, like many proteases, subject to autocatalytic proteolytic degradation which follows second order kinetics subject to product inhibition (Jespersen et al. 1986, Thrombosis Research 41, 395-404).
In the early 1960's it was established that plasmin can be stabilized at acidic pH, or alternatively at neutral pH provided an amino acid such as lysine is present. Nevertheless, autolytic cleavage after Lys104, Arg189 and Lys622 (numbering relative to Lys-plasmin) were reported even when plasmin is stored at pH 3.8 (WO 01/36608). When plasmin is stored at the even lower pH of 2.2, non-autolytic acid cleavage occurs between Asp-Pro (D-P) at positions Asp62, Asp154 and Asp346 (WO 01/36608). This illustrates that pH can be lowered to a point where no apparent autocatylic degradation occurs anymore but at which acid hydrolysis is becoming a factor of destabilization. No information is present in WO 01/36608 as to which peptide bonds in plasmin are vulnerable to (autocatalytic) hydrolysis at neutral pH. Known stabilizers of plasmin include glycerol, sufficiently high ionic strength, fibrinogen and ε-aminocaproic acid (EACA), as disclosed by Jespersen et al. (1986, Thromb Res 41, 395-404). Lysine and lysine-derivatives (such as EACA and tranexamic acid) and p-aminomethylbenzoic acid (PAMBA) are some further known stabilizers (Uehsima et al. 1996, Clin Chim Acta 245, 7-18; Verstraete 1985, Drugs 29, 236-261). U.S. Pat. No. 4,462,980 reported on the formation of plasmin aggregates contributing to plasmin degradation despite storage at acidic conditions. A solution to this problem was provided in U.S. Pat. No. 4,462,980 by means of adding a polyhydroxy compound. Other ways of stabilizing plasmin include the addition of oligopeptidic compounds (e.g. U.S. Pat. No. 5,879,923). Alternatively, the catalytic site of plasmin can be reversibly blocked by means of derivatization, e.g. acylation (EP 0009879). Pegylation of plasmin has also been suggested as a means to stabilize the enzyme (WO 93/15189).
A number of plasmin variants other than truncated forms of plasmin have been described and include a chimeric microplasmin (WO 2004/045558) and variants with a point mutation at the two-chain cleavage site (U.S. Pat. No. 5,087,572) or at a catalytic triad amino acid (Mhashilkar et al. 1993, Proc Natl Acad Sci USA 90, 5374-5377; Wang et al., 2001, J Mol Biol 295, 903-914). Wang et al. (1995, Protein Science 4, 1758-1767 and 1768-1779) reported an extensive series of microplasminogen mutants at amino acid positions 545, 548, 550, 555, 556, 558, 560-564, 585, 740 and 788. A double mutant wherein cysteines at amino acid positions 558 and 566 were substituted for serines was reported by Linde et al. (1998, Eur J Biochem 251, 472-479). Takeda-Shitaka et al. (1999, Chem Pharm Bull 47, 322-328) refer to a plasmin variant with reduced activity, the variation involving the substitution of alanine at amino acid position 601 to threonine. All amino acid positions referred to above are relative to Glu-plasminogen starting with Glu at amino acid position 1. A non-cleavable plasminogen variant (cleavage between heavy and light chain impaired) is described in WO 91/08297. Dawson et al. (1994, Biochemistry 33, 12042-12047) describe the reduced affinity for streptokinase of a Glu-plasminogen variant with a Glu instead of Arg at position 719 (R719E). Jespers et al. (1998, Biochemistry 37, 6380-6386) produced in an Ala-scan the series of phage-displayed microplasminogen single-site mutants H569A, R610A, K615A, D660A, Y672A, R712A, R719A, T782A, R789A, and found that arginine at position 719 is key for interaction with staphylokinase; the D660A mutant was not further characterized due to very low expression; only the R719A mutant was additionally produced in soluble form. None of the mutants showed a gross change in proteolytic activity (substrate S-2403). Jespers et al. (1998) also included an active site mutant S741A in their analysis; the crystal structure of this mutant is disclosed in Wang et al. (2000, J Mol Biol 295, 903-914). In further attempts to unravel the streptokinase/plasminogen interaction sites, Terzyan et al. (2004, Proteins 56, 277-284) reported a number of microplasminogen mutants (K698M, D740N, S741A) in an already mutated background (R561A), the latter prohibiting proteolytic activation of plasminogen and thus prohibiting formation of active microplasmin (which would complicate the study of the contact-activation mechanism of the streptokinase-microplasminogen complex). Terzyan et al. (2004) further mention an “inadvertent” triple mutant R561A/H569Y/K698M apparently functionally indifferent from the double mutant R561A/K698M. Wang et al. (2000, Eur J Biochem 267, 3994-4001), in studying streptokinase/plasmin(ogen) interaction, produced a set of microplasminogen (amino acids 530-791 of Glu-plasminogen) mutants in a Cys536Ala and Cys541Ser background. These mutants include the R561A mutation as described above (Terzyan et al. (2004)) as well as R561A/K698G, R561A/K698A and R561A/K698Q double mutants. In the same C536A/C541S background, single K698G and K698A mutations were introduced also, of which the K698G was not characterized further due to difficulties with purification. The above studies aimed at obtaining a better understanding of the characteristics of the plasminogen/plasmin molecule and did not report any clinical usefulness or benefit or putative clinical advantages of the plasminogen/plasmin mutants. Peisach et al. (1999, Biochemistry 38, 11180-11188) succeeded in determining the crystal structure of microplasminogen containing the M585Q, V673M and M788L mutations.
Nguyen & Chrambach (1981, Preparative Biochem 11, 159-172) reported the presence of “a minor and unidentified protein component” of 10.0 kDa based on reducing SDS-PAGE of a crude commercial preparation of urokinase-activated plasmin (Homolysin). The differences in autolysis of human plasmin depending on pH have been described in detail by Shi &Wu (1988, Thrombosis Research 51, 355-364). Ohyama et al. (2004, Eur J Biochem 271, 809-820) proposed the use of non-lysine analog plasminogen modulators in treatment of cancer due to the enhancement of plasmin autoproteolysis by such compounds which results in the enhanced formation of angiostatins (in the presence of the plasminogen activator urokinase). Table 3 of Ohyama et al. (2004) lists as many as 15 cleavage sites within plasmin subjected to autoproteolyis-enhancing compounds. In discussing their observations in view of prior investigations, it would seem that the autoproteolyis-enhancing compounds are more or less selectively enhancing proteolysis of the B/light-chain whereas minimum degradation of both A/heavy- and B-chain was found in the absence of the autoproteolyis-enhancing compounds.
It is clear that none of the above methods/variants solves the problem of providing a plasmin stabilized at the molecular level. The provision of a plasmin variant (or of a corresponding plasminogen variant from which plasmin can be derived) with a catalytic domain intrinsically resistant to autocatalytic degradation would be a significant step forward towards efficient and safe long-term storage as well as towards efficient and safe therapeutic use of plasmin such as in thrombolytic therapy or in the induction of posterior vitreous detachment or vitreous liquefaction in the eye.